Which of these numbers is prime? ${13,\ 30,\ 49,\ 65,\ 87}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 30 are 1, 2, 3, 5, 6, 10, 15, and 30. The factors of 49 are 1, 7, and 49. The factors of 65 are 1, 5, 13, and 65. The factors of 87 are 1, 3, 29, and 87. Thus, 13 is a prime number.